Sorry, I know I'm reading into your statement, but your reaction is very stereotypical 'liberal', aka 'thinks too much'. I do agree with you in that people do have biases, but there are several levels to this, perhaps. The biases you speak of are surface level ones, but 'gut' things are at a deeper level, at least how I'd define things.
I'd say there is a learning curve associated with 'gut' instincts. If your parents were good ones, you'd have acquired such knowledge earlier than I ever did, which to say was much later than ideal... But once you have dealt with enough assholes and full blown psychopaths, your 'gut' is better at picking up the subtle clues.
Then you can start seeing thru the surface impressions easier - I'd say the people you speak of just hadn't had enough adversity in their life yet - and from a BS New Age point of view, that's what allowed this guy to install himself in their lives, to provide them with some life lesson they needed to go through. I'm certainly not blaming the victim - it's just that if they don't take away a useful life lesson then they're missing out on extracting some value from the experience.
I empathize with the people in the article immensely. I'd just say they were too trusting, too 'liberal' (in all the associated stereotypes) and their 'gut' was dumb/inexperienced. The psychopaths/Machiavellians I've had to deal with were not involved in my life as much as a co-tenant would be, but they were close enough. And I consciously picked up little clues to their essential nature over time, but kept not integrating them into a larger more comprehensive picture till it all became a crisis. It is acting on those clues fast enough that is key, and that requires learning through experience.
tldr; there are book smarts and there are street smarts. Both have utility and to rely heavily on one and discount the other is unwise. Street smarts involve training your gut to deal with the jackasses and worse you will encounter in life.